Composition-, Size-, and Surface Functionalization-Dependent Optical Properties of Lead Bromide Perovskite Nanocrystals

The photoluminescence (PL), color purity, and stability of lead halide perovskite nanocrystals depend critically on surface passivation. We present a study on the temperature-dependent PL and PL decay dynamics of lead bromide perovskite nanocrystals characterized by different types of A cations, surface ligands, and nanocrystal sizes. Throughout, we observe a single emission peak from cryogenic to ambient temperature. The PL decay dynamics are dominated by surface passivation, and a postsynthesis ligand exchange with a quaternary ammonium bromide (QAB) results in more stable passivation over a larger temperature range. The PL intensity is highest from 50 to 250 K, which indicates that ligand binding competes with the thermal energy at ambient temperature. Despite the favorable PL dynamics of nanocrystals passivated with QAB ligands (monoexponential PL decay over a large temperature range, increased PL intensity and stability), surface passivation still needs to be improved to achieve maximum emission intensity in nanocrystal films.

S2 120 °C. Next, the temperature was lowered to 65 o C and a methylamine solution (0.170 mL) was injected, followed by the swift injection of benzoyl bromide solution (70 µL) in ODE (500 µL, which had previously been degassed for an hour at 120 °C and stored in glove box). The reaction mixture was quenched by the addition of 5 mL of toluene after 30 s and NCs were collected by centrifuging the crude solution at 4000 rpm for 10 min. The supernatant was discarded, and the precipitate was redispersed in 5 mL of toluene.
iii) Synthesis of FAPbBr3 NCs: 76 mg of lead (II) acetate trihydrate, formamidinium acetate (40 mg), 2.5 mL of OA, 0.025 mL of OLAM, and 5 mL of ODE were mixed in a 25 mL 3-neck round-bottom flask and dried under vacuum for 1 h at 120 °C. Then, the temperature was lowered to 75°C under N2, and a solution of benzoyl bromide (70 µL) in ODE (500 µL, which had previously been degassed for an hour at 120 °C and stored in glove box) was swiftly injected. After 30 s, the reaction mixture was cooled down in an ice−water bath. Thereafter, 5 mL of toluene was added to the crude solution, then it was centrifuged at 5000 rpm for 10 min. The supernatant was discarded, and the precipitate was redispersed in 5 mL of toluene for further use.

Synthesis of Cs-Oleate capped CsPbBr3 NCs:
Cs-oleate capped CsPbBr3 NCs were synthesized following our previously reported procedure using standard Schlenk line techniques. 2 Briefly, lead (II) acetate trihydrate (76 mg) cesium carbonate (16 mg) and ODE (10 mL) were combined in a 25 mL 3-neck flask equipped with a thermocouple and a magnetic stirrer. The reaction mixture was degassed for 5 min at room temperature and then for one hour at 115 °C.
Then, a ligand mixture containing oleic acid (1.5 mL, previously degassed for an hour at 120 °C and stored in glove box), didodecylamine (1.25 mmol, 443 mg) dissolved in 1 mL of anhydrous toluene was rapidly injected under nitrogen. After the complete dissolution of the metal precursors, the temperature was decreased to 70 °C and a solution of benzoyl bromide (50 µL) in anhydrous toluene (500 µL) was swiftly injected. After 60 s, the reaction mixture was cooled down by using a water bath. Then 20 mL of an ethyl acetate and toluene mixture (with a ratio of 6:1) was added into the crude solution to destabilize the colloids followed by centrifugation at 6000 rpm for 10 min. Finally, the supernatant was discarded, and the precipitate was redispersed in toluene.

S3
Synthesis of QAB capped CsPbBr3 NCs: QAB capped CsPbBr3 NCs were prepared following a previously reported ligand exchange strategy. 3 Briefly, Cs-oleate capped NCs were prepared following above mentioned procedure 2 and the crude reaction mixture containing the CsPbBr3 NCs (3 mL) was treated with an anhydrous toluene solution containing the didodecyldimethylammonium bromide salt (2 mL, 0.025M) and the mixture was vigorously stirred for 1 min. Thereafter, the NCs were washed by addition of 15 mL of ethyl acetate followed by centrifugation at 6000 rpm for 10 min and re-dispersion in toluene.        Table S2. Parameters of the exponential fitting of the PL decay traces, and the average PL lifetimes of MAPbBr3 NCs obtained from the traces in Figure 2c: Table S3. Parameters of the exponential fitting of the PL decay traces, and the average PL lifetimes of FAPbBr3 NCs obtained from the traces in Figure 2c: Where ' is the inhomogeneous broadening, )* and +, describe the coupling strength to the acoustical and LO phonons, respectively, and +, is the energy of the LO phonon. 5 is the Boltzmann constant. The values obtained from the fitting shown in Figure S6 are summarized in Table  S4.     XRD spectra recorded at different temperatures for mixed ligands capped CsPbBr3 NCs. We observed the appearance of additional sharp peaks from 140K down to 20K, which disappeared when the NCs film was heated back to 300K. We attribute these peaks to the formation of a thin ice layer at the sample surface after few hours of measurements at a pressure of at 10 -6 mbar, or to frozen organics. S11 Table S5: Thermally induced variation in the cell parameters of mixed ligands capped CsPbBr3 NCs